Composite materials are finding broader acceptance and application in industry, especially in modern military and civilian aircraft. High temperature thermoplastic, fiber reinforced composite material possesses high strength, high stiffness, and desirable damage tolerance characteristics. Thermoplastic composites can be made by several low cost manufacturing processes. But, thermoplastic composite material poses challenges for structural repairs, especially field repairs, which are essential if the thermoplastic composites are to find wide scale use. For example, airplanes are too valuable and too expensive to keep grounded for any extended length of time, especially commercial aircraft where grounding for hours has significant commercial repercussions.
Traditionally, field repair of composite structure, whether metal or composite, has used bolted or bonded paths. In the bolted approach, a cover plate is mechanically fastened around the damaged area. The fasteners transfer the loads into the patch, but patches and fasteners create stress concentrations and require the drilling of holes of close tolerance diameter in the parent structure that degrade its performance. Accordingly, bolted repair is unacceptable (or discouraged) for thin skinned or highly-loaded structure. A bolted repair may also adversely impact the aerodynamics or radar signature of an aircraft.
Bonded repairs involve removing the damaged area to prepare an appropriate interface for the patch. The damaged material is usually scarfed away to prepare a tapered or stepped depression in the laminate using the equipment and the processes described in U.S. Pat. No. 4,987,700 and 5,207,541, which we incorporate by reference. In some circumstances, the repair hole might be shaped rather than round, as shown in U.S. Pat. No. 4,916,880 and 4,978,404, which we also incorporate by reference. Shaped inserts for the patch material often permit load transfer better than plug patches.
In a bonded repair, the cutout is filled with an appropriate uncured, resin prepreg materials which subsequently must be cured or bonded to the parent structure. As appropriate, adhesives are used with the prepreg materials and care is taken to provide load paths in the fiber reinforcement, if possible. These preparations are described in the patents referenced above. Curing or bonding requires heat, which we usually furnish with a thermal diffusion heat blanket, like that described in U.S. Pat. No. 5,442,156, which we also incorporate by reference. Other heaters, of course, might be used. The patch material might be the same material as the parent structure, or, in the present invention, because of the pin-reinforcement, might be a different material, such as a graphite/epoxy patch in a K III B/carbon fiber thermoplastic polyimide parent structure. The ability to use different raw materials in the patch than those used in the parent structure enhances the suitability the present invention for field repair by broadening the candidate patch materials or by reducing the necessary field stores inventory.
Conventional patches usually are selected from materials identical with the materials in the parent structure being repaired. While bonded patches provide more efficient load transfer than bolted repairs and the flush finish is less intrusive from the aerodynamic and signature standpoints, the quality of a bonded repair, like the quality of the parent structure, is highly dependent on the absolute age and the aging history of the prepreg materials used in the patch. In addition, the quality of a conventional bonded patch is also dependent on the interface preparation, on the curing or binding process parameters (including the thermal uniformity), and, finally, on the talent and artistry of the workers preparing the patch. Needless to say, the combination of significant variables makes it difficult to achieve consistent structural performance in the repaired structure.
Conventional bonded repairs are currently an unattractive option for thermoplastic structures, especially those using high temperature thermoplastics. Using thermoplastic adhesives and thermoplastic repair materials would generally damage the parent structure. While thermoplastics can be reheated to their consolidation temperature and reformed a number of times, at the temperature required to cure the high temperature thermoplastic repair material, the parent structure would begin to soften and reconsolidate. Often the parent components are originally consolidated at high pressures often using complex tooling to achieve critical shaping and dimensions necessary for performance. Reconsolidation under field conditions would be impractical. Loss of the critical dimensions could be catastrophic. Thermoplastics are also difficult to repair using adhesive bonding because the thermoplastic resins often are resistant to solvents and adhesives. As a result, conventional bonded repairs to these thermoplastic structures produce inferior performance.
For advanced thermoplastic composites to be adopted widely for emerging, industrial (especially aerospace) structural applications (the uses for which the materials are designed), industry must improve the quality of field repairs and reduce the extensive "logistics tail" typically associated with composite structures. Repair approaches must be identified and developed which strike a balance between the time and resources associated with their use and the structural and aerodynamic performance they provide.
Previous work has concentrated primarily on improving the performance of bonded repairs. Work done to enhance bond strength has centered around improving the prebonding surface preparation processes. Many different approaches have been pursued including plasma spray, chemical etch, and corona treatment (electrical discharge over part-surface). These methods are neither user friendly nor readily field applicable. They often fail to produce improvements in repair strength consistently. Other joining processes attempted for thermoplastic repair include welding, which entails heating the interface between the parent structure and patch to a temperature high enough to cause the materials to both melt and to reconsolidate together. For high temperature thermoplastics, welding requires elevated temperatures and high pressure to be applied to the parts which usually makes the process unattractive for field repair. We describe welding operations in U.S. Pat. Nos. 5,313,037, 5,444,220; 5,486,684; 5,660,669; and 5,710,412. Although not necessarily field repair processes, we incorporate these patents by reference.
Improvements are still required for bonded field repairs of aircraft to produce adequate patches easily, reliably, and reproducibly to return the aircraft quickly to service (at least temporarily). We have discovered that incorporating Z-pins in the repair provides structural improvement and permits broader processing windows for otherwise critical preparation variables. Z-pin patches also permits field repair of advanced thermoplastic structure using otherwise incompatible thermosetting graphite/epoxy patches.